In recent years there has been a growing recognition of the importance of maternal health and the uterine environment in the later health of children. Many women experience chronic or acute health conditions while pregnant, and a few studies have linked certain conditions with increased pediatric cancer risk. Maternal health care may further involve the use of medications to treat a preexisting condition or a condition that develops during the prenatal period. It is well-established that maternal intake of diethylstilbestrol (DES) causes cancer in offspring, however for most drugs, the evidence on safety in pregnancy is limited. The aim of this study is to examine common maternal health conditions and prescription drug use in pregnancy in relation to later childhood cancer development. Specific aims: (1) Create a linked database of all childhood cancers in Denmark (1977-2012) with information on maternal comorbidities and medication use. (2) Examine the relation between maternal comorbid conditions and childhood cancers. (3) Examine the relation between maternal prescription drug use in pregnancy and childhood cancers. Design and methods: A national case-control study using national registries in Denmark, using data from an existing study on childhood cancers in Denmark (n=537,000). Maternal comorbidity information will be available from the Danish National Patient Register (1977-2012), and prescription drug use in pregnancy from the Danish National Prescription Register (1994-2012). Data on demographic, gestational, and other factors will be available from other national registers. Implications: the findings will add evidence on childhood cancer risk among children of mothers with certain health conditions or who took specific medications during pregnancy. The results will indicate if more active monitoring and screening is needed to reduce the potentially increased risk for the unborn child, and can help guide recommendations on medication use in pregnancy.